1. Field of the Invention
The present invention relates to non-tacky conformable material suitable for use as a void filling, electrical stress controlling material. Specifically, it relates to nontacky core-loadable epihalohydrin polymer compositions which exhibit effective stress control when used at the edge of a semi-conductive shield in a termination either alone or in conjunction with a high dielectric constant tube.
1. Description of the Art
Heat-recoverable rubbery tubular articles and elastically recoverable tubular articles are widely used to cover power cables or conduits. Such articles are useful where insulation, sealing and/or encapsulation are required. A typical elastically recoverable article is a rubber insulating sleeve which is provided with an inner support or "core" which holds the article in a deformed shape. The tube is placed over a cable, and the core is unwound, and removed. The article then recovers toward an original shape.
Various compositions have been used in coordination with such articles to provide electrical stress control and sometimes to bond them to various types of pipe and tubing, e.g., mastics, tapes, and greases as in U.S. Pat. Nos. 4,017,652, and 4,383,131.
U.S. Pat. No. 4,363,842 discloses elastomeric pre-stretched tubes formed from EP or EPDM with a variety of fillers including carbon black, conductive particles and the like, including carbon black, and aluminum flakes. Barium titanate is disclosed for providing stability of permittivity over a wide range of electrical stresses; barium titanate does not provide higher permittivity to the elastomeric tubes.
Heat-recoverable articles may also be provided on an inner tube, which breaks and allows the elastomeric member to recover. These articles are heated with a hot air gun or torch, to initiate recovery.
Both types of articles may recover around the cable joint or termination (or other substrate) tightly in areas, and have air pockets between the cable and the substrate about which it is recovered in other areas, such as a semiconducting layer edge, which is highly undesirable. Sealing compositions, such as greases, adhesives, and the like have been used with these articles to reduce or eliminate such air pockets. Many of such compositions have been adhesives which have bonded between the recoverable articles and the substrate about which it is recovered.
EP Patent 0424 090 B 1 discloses an elastomeric covering for connections in electrical cables having a conformable material interposed between the prestretched tube and the core, whereby when the core is removed, the elastic forces change the shape and dimensions of the material to correspond to the underlying surface. No electrical properties are disclosed, and a broad range of materials are simply listed.
U.S. Pat. No. 4,378,463 discloses an adhesive for use in heat recoverable products in areas of electrical stress. The adhesive comprises an epihalohydrin polymer, a tackifier, a catalyst, and a particulate filler. The composition bonds the heat-recoverable article to the substrate, and also provides electrical stress relief. No polymer blends are disclosed.
However, the above composition has some disadvantages. First, it must be provided separately, and cannot be preloaded into the splice or termination as the composition will adhere thereto or to the core and either prevent its removal when it is time to initiate recovery or deform and fracture during removal, causing the formation of large air pockets.
Applicant has now discovered that a non-tacky stress control material containing an epihalohydrin polymer, and an insulative polymer having a tan .delta. of less than 1, and having certain fillers provides unique electrical properties for use in stress control, i.e., excellent permittivity. Such compositions exhibit synergism between the combination of polymers and the fillers not seen when only one type of polymer is used with the same fillers. The materials have significantly improved results on electrical properties such as permittivity, alternating current voltage withstand tests, impulse withstand, and/or tan .delta. values.
Further, compositions of the invention may be aged in cable oil and neither swell nor lose their electrical properties.
The compositions may be used alone inside the insulator of an electrical cable, and are suitable for preloading in an elastically shrinkable tube such as a splice cover or termination supported on a removable rigid core, or underneath a pre-stretched elastomeric or heat shrhinkable stress control tube.